Reference is made herein to tyres fitted to the wheels of different types of vehicles (motor cars, trucks, motorcycles, etc.).
The functional characteristics of a tyre depend on its state of radial deformation (flattening towards the ground under load) and affect the dynamic behaviour of the vehicle.
Monitoring the state of radial deformation of vehicle tyres concerns what has become a priority issue in the modern-day automotive trade, and that is, vehicle roadworthiness and safety (correctly inflated tyres allow better control, better road grip and optimized braking distances) but not only that. It also regards the key issues of energy saving and pollution: correctly inflated tyres allow major savings not only in terms of longer tyre life but also in terms of lower fuel consumption (since rolling friction is reduced). That in turn means less pollution due to vehicle emissions which can be reduced by many percentage points.
This must be viewed against an actual setting where motorists the world over seem to care very little about the state of wear of the tyres of their vehicles: statistics indicate that 90% of the vehicles currently in circulation worldwide have insufficiently inflated tyres and that in 50% of these, tyre pressure is below the limit for correct use (approximately 25% less than the nominal tyre pressure).
The main reason is that tyres are subject to a slow but inexorable loss of pressure due to the natural molecular migration of gas through the tyres walls.
It should be noted that a tyre underinflated by 20% of its nominal pressure is the cause of higher fuel consumption and hence higher exhaust gas pollution (approximately 3%) and has a working life reduced by 30%, with a corresponding increase in pollution due to harmful particulates (from tyre wear) and higher costs for the vehicle owner.
If the social costs arising out of accidents due to the use of vehicles with underinflated tyres are added to this, it is clear that research for effective and easy to use devices for monitoring the state of radial deformation of tyres is a key factor in the future development of the industry.
The state of radial deformation of a tyre under load is proportional to the difference between the pressure of the gas inside the tyre and the atmospheric pressure of the environment outside it. This is also known as state of tyre inflation.
Thus, correct monitoring of the state of radial deformation of the tyre inevitably implies measuring the absolute pressure inside the tyre and the pressure of the atmosphere outside it.
In light of this, TPMS is the acronym for Tyre Pressure Monitoring System.
Research in industrial applications in this field has been in progress for several decades but, owing to the complexity of the subject, the first significant applications have been brought out only in the last few years, thanks also to United States federal legislation which has made it compulsory to install TPMS on new vehicles.
Today, TPMS are one of the most important and promising fields of the automotive sector and, more generally, of the transport vehicle sector. Given the size of the vehicle market, in terms of quantities and turnover, it may be considered one of the most important business opportunities of the present and at least for the next ten to fifteen years.
All the major companies in the automotive industry and in the industry of automotive components are competing to develop safe, functional and cost-effective solutions applicable on a large scale to vehicles of different kinds.
TPMS systems of both “direct” and “indirect” type have been developed. TPMS systems are said to be “direct” when they measure the pressure inside each tyre using appropriate pressure sensors, with the aid of temperature sensors, if necessary, to take into account the compressibility of the gas, while “indirect” TPMS systems infer the state of tyre inflation from the values of other measured quantities, such as, for example, rolling speed.
Direct systems are much better performing and are therefore preferred even if their cost is higher than that of indirect systems.
Generally speaking, state of the art, direct TPMS systems are equipped with pressure sensors which measure the absolute pressure inside the tyre and assume a fixed absolute value (for example 1000 mbar) for the pressure outside, and thus do not measure the radial deformation of the tyre.
Prior art TPMS systems of the direct type normally comprise a sensor unit (comprising at least one pressure sensor and, if necessary, at least one temperature sensor) associated with a unit for generating and transmitting electromagnetic signals, suitable for transmitting to the outside indications regarding the state of inflation of the tyres and equipped with power supply systems (for example batteries) to be installed on each wheel to be monitored, and at least one receiving unit for receiving the electromagnetic signals from the sensor units mounted on the wheels and providing the vehicle driver with indications.
A first prior art solution comprises modules consisting of electronic pressure sensors, associated with radio transmission apparatuses and respective antennas, to be mounted inside each tyre (or fastened either to the inside groove of the wheel rim using a metal strip, as schematically illustrated in FIG. 1, or to the protrusion inside the inflation valve, as schematically illustrated in FIG. 2).
Owing to their inherent properties, these modules are relatively large in size and weight, as described below.
Architectures which involve fitting relatively large and heavy objects inside the tyres are suitable only for original equipment applications (that is, equipment which is factory-fitted on the vehicles by the manufacturers) and are certainly not suitable for retrofitting or after market applications (that is, for fitting on vehicles currently in circulation, whose number is approximately twelve times higher than the number of new vehicles manufactured each year).
That means automatically excluding a sufficiently rapid spreading of these systems to a significant number of vehicles, thereby losing a major business opportunity (which, at best, would be limited to new vehicles only) and excluding for a long time to come the acquisition of real “social” benefits such as those that would result from reducing pollution emissions guaranteed by proper use of tyres by a significant number of motorists.
These prior art solutions, based on sensors to be installed inside the tyre, have several inherent disadvantages, which are difficult, if not impossible to eliminate. Described below are the main and most common problems encountered.
A first problem is the following.
Since tyre monitoring makes sense if it allows unsafe vehicle driving conditions to be detected in good time, the state of inflation of each tyre must be measured and signalled to the driver without delay: thus, the pressure and temperature sensors must be permanently active and cannot be disabled for long periods (in practice, they cannot be disabled for more than a few seconds). The sensors, together with the signal generating and transmission system, are the main power consumers.
For this reason, the traditional systems consume a considerable amount of power and the apparatuses provided for each tyre for measuring pressure and, if necessary, temperature are usually powered by batteries, whose size and weight constitute the main part of the systems since they must have a working life of several years.
The problems typical of these devices are the following:                a. the systems do not work in the absence of power supply;        b. the systems need a continuous supply of power;        c. if the systems are activated only for limited periods in order to save energy, the warning signal may be given with some delay after the event to be detected;        d. owing to the high number of radio signals transmitted per unit time, these systems produce a high level of electromagnetic disturbances.        
Further, in most of the systems based on these architectures, the apparatuses to be mounted inside the tyres are relatively large in size and, above all, in weight, being objects of considerable volume and weighing between 20 and 40 g, due mainly to the size and weight of the batteries allowing little room for improvement in this respect. The inclusion of these objects in the tyres creates considerable problems of component adaptation from the industrial viewpoint and is therefore a source of costs but above all of risks due to the fact that wheel rotation generates a centrifugal acceleration which easily exceeds 20,000 m/s2: in these situations, a weight of 30 g translates as a force of 600 N, and unfortunately, it is not infrequent for the fastening of the apparatus to break, turning the apparatus into a bullet which may pierce the tyre and cause extremely serious and often irreparable damage.
A second problem is the following. The inclusion in the tyres of relatively large objects fastened to the rim leads to complications linked to the fitting and removal of the tyres since the objects themselves can be easily damaged or broken during such operations.
A third problem is the following. The electronic pressure sensors of these apparatuses are necessarily of the absolute type, that is to say, they are unable to measure the difference between the absolute pressure inside the tyre and the absolute atmospheric pressure outside it since they are not physically in communication with the environment outside the tyre. The static and dynamic behaviour of a tyre, on the other hand, depends on that pressure difference and not on the absolute internal pressure. It follows that the measurements performed with these apparatuses are inherently subject to error. The error may even be significant since, under equal conditions of internal pressure, the atmospheric pressure may vary by up to 30%, which means that the variations of the pressure differences may exceed 10% of the normal inflation pressures of most vehicle tyres.
In this regard, reference is made to the article presented at the SAE World Aviation Congress, Advances in Aviation Safety, Sep. 8-11, 2003, Montreal, QC, CANADA, Session: Maintenance Safety II, available from http://www.sae.org/technical/papers/2003-01-2980), entitled “Tire Monitoring Systems Design: a Novel Approach” (SAE paper n. 2003-01-2980).
Reference is also made to the paper presented at the SAE 2004 World Congress & Exhibition, Mar. 8-11, 2004, Detroit, Mich., USA, Session: Vehicle Sensors & Actuators, available as SAE publication (SAE Paper Title: “On Tire Monitoring Systems Temperature Compensation”, Document Number: 2004-01-1110) from “http://www.sae.org/servlets/productDetail?PROD_TYP=PAPER &PROD_CD=2004-01-1110”.
A fourth problem is the following. Architectures which involve fitting relatively heavy objects inside the tyres require dynamic rebalancing of the wheel. This is usually done with counterweights made of lead, a material which will soon be prohibited by anti-pollution regulations. To overcome this problem, in original equipment for vehicles to be fitted with the sensors, special wheel rims are manufactured with built-in weight to balance the weight of the sensor. This involves managing different rim versions (with and without sensor) for every rim design, thereby increasing costs. Moreover, these architectures are suitable only for original equipment applications and certainly do not lend themselves to retrofitting or after-market applications.
A fifth problem is the following. The systems based on these traditional architectures measure the absolute internal pressure of a tyre and, if necessary, a temperature whose real value is intermediate between the temperature of the gas inside the tyre and the temperature of other components of the wheel assembly to which the temperature is close or connected. The pressure sensor normally has a fixed value as reference pressure.
A sixth problem is the following. These sensors, usually electronic, are connected to apparatuses for processing and transmitting the data to the outside. The external receiver is connected to a processing control unit which in turn transmits a signal to a display device normally mounted on or built into the vehicle dashboard. The transmission of the signal from the apparatus fitted to each tyre to the external receiver occurs through at least one transmitting antenna and one receiving antenna. Therefore, these systems are particularly complex and expensive since they include at least one cabling.
Prior art devices of a different type are based on units to be applied to individual wheels on the outside of the tyre instead of on the inside.
More specifically, patent document US2001/0023613 discloses a tyre pressure measuring device, having a gas inside the valve, separated from the space inside the wheel by a movable wall associated with an indicator which is movable on a graduated scale to provide a reading. The gas inside is pre-loaded in such a way as to oppose the pressure inside the tyre to place the indicator in equilibrium.
Patent document WO2007/095199 discloses a tyre pressure gauge comprising a pressure sensor located inside the valve and a display to provide a visible indication of the pressure measured by the sensor.
These systems, however, are not practical to read and do not provide the vehicle driver with timely indications (that is to say, the driver must take the trouble of going to read them, involving the inconvenience of having to take periodic readings, even in situations where tyres are correctly inflated).
In particular, the devices of this kind include devices which can be screwed onto the inflation valves in place of the valve cap and some of which are much smaller in size and weight than the traditional apparatuses described above (it is not even necessary to re-balance the wheels after installing the devices). These devices therefore solve in whole or in part some of the problems described above (FIG. 3).
Since they are easy to access, however, these devices are also easy to steal and must therefore be provided with antitheft systems, meaning that they can be removed from the valve only with suitable tools.
That may make it complicated to take periodic action on the tyres to restore the correct pressure.
Further, their axial length is greater than that of the standard valve cap and that means, in some cases, that it is difficult to meet the specification requiring the valve not to protrude from the rim.
In any case, prior art devices have some drawbacks in common, as described below.
Every device to be applied to a wheel is usually composed of two or more components to be interfaced mechanically, pneumatically and electrically. For example, devices fastened to the protrusion inside the inflation valve need at least one mechanical interface and one electrical interface when they use the valve stem as an antenna, or devices fitted on the outside of the tyre in place of the valve cap need at least one mechanical interface and one pneumatic interface, and so on. As is known, each interface means higher production and assembly costs, risk of faults, and non-optimized dimensions.
In light of this, the prior art devices do not meet the need to provide a system for monitoring the state of radial deformation of a tyre which is at once not only effective and reliable but also light in weight, small in size, inexpensive and easy to use.